pit. The Empress attacks Earth with a ship resembling a Christmas star. The Doctor attempts to offer a peaceful solution but the Empress refuses. The Doctor then remotely detonates explosive baubles used by the Santas at the walls of the base, flooding the pit with water from the Thames. The Doctor is prepared to die, but Donna urges him to escape with her, just as the Empress teleports to her ship to try to escape. However, the ship is out of huon energy, and human forces destroy the ship. The Doctor asks Donna to travel with him. She declines, but suggests he needs a companion to keep his temperament in check. Production Russell T Davies had the idea for this episode from the very beginning of his association with the programme, and he planned to air it in Series Two. With the public announcement of two Christmas specials and the private knowledge of Billie Piper leaving at the end of Series Two, Davies decided to elevate this story to the Christmas special, not introducing the new companion immediately, and filling the slot with "Tooth and Claw". The end of "Doomsday" is featured as part of the pre-title sequence, although the scene was actually refilmed. In his online podcast commentary for the episode, David Tennant explained that this was due to a change in lighting supervisors, and the one hired for this episode liked to light the TARDIS interior differently; the scene therefore had to be refilmed in order to match. The Doctor Who logo in the opening credits has been slightly redesigned from the previous one, with more background detail and flare on the "lozenge" that the words "Doctor Who" sit on. For legal reasons, the production team made obviously fake banknotes for the scene where money comes flying out of a cashpoint. The £10 notes feature the Doctor's face and the phrases "I promise to pay the bearer on demand the sum of ten satsumas" and "No second chances — I'm that sort of a man". The text is a reference to the Doctor's actions and dialogue near the end of "The Christmas Invasion". There were also £20 notes featuring producer Phil Collinson. These had the phrase "There's no point being grown up if you can't be a little childish sometimes" printed on them, misquoting the line originally spoken by the Fourth Doctor (Tom Baker), in Robot, "There's no